1. Field of the Invention
The present invention relates generally to circularly movable cranks, and in particular, to crank arm radius adjusting mechanisms useful for pedal powered vehicles such as bicycles and stationary exercise cycles.
2. Discussion of the Prior Art
Devices for the adjustment of a crank arm radius or stroke length for a bicycle driving crank have been known in the art for a considerable period of time. A summary of such prior art is provided in the specification of U.S. Pat. No. 4,648,287, issued Mar. 10, 1987 to J. Preskitt for "Pedal Stroke Adjuster for a Bicycle or Exercise Machine".
One early version of such a device can be noted in U.S. Pat. No. 2,533,011, issued Dec. 5, 1950 to W. Hill for "Bicycle Pedal Adapter". In this device, a special collar or bracket is configured to slidably engage over a distal end of a standard bicycle crank arm to extend substantially radially inwardly from the typical threaded pedal engaging hole found proximate to the distal end of the crank arm. Bolt means are inserted through appropriate holes in the collar or bracket and into the threaded hole of the crank arm to secure the collar or bracket to the crank arm. The collar or bracket includes a flange portion having holes therethrough disposed to be substantially adjacent a radially inwardly extending portion of the crank arm. The pedal is engaged to the flange portion through its holes. Each pedal of the pair normally employed on a bicycle or stationary exercise cycle is so mounted to respective flanges of respective collars or brackets such that they effectively act on their respective crank arms at equally reduced radii from the center of rotation of the drive crank of the bicycle. It would appear from the teaching of the Hill patent that a single radius of adjusted stroke length is available from a particular set of collars or brackets, and that other radii would require the installation of different collars or brackets.
The above-mentioned patent to Preskitt, U.S. Pat. No. 4,648,287, provides a device having a range of discrete adjustment radii at which the pedals of the bicycle may be disposed to act on respective crank arms. This is accomplished by providing an adjustment frame extending substantially radially inwardly from the normal pedal engagement hole of the crank arm, located proximate to its distal end. The adjustment frame is provided with appropriate spacers to accommodate attachment of the frame to a crank arm that diverges from the plane of rotation of the crank along the length of the crank arm to its distal end. With said spacers emplaced, a first end of the adjustment frame is coupled to the distal end of the crank arm by bolt means extending through the pedal mounting hole at the distal end of the crank arm. The obverse end of the adjustment frame is held to a radially inwardly extending portion of the crank arm by a bracket encircling the crank arm. A block, containing a hole through which the pedal is engaged, is slidably coupled to the adjustment frame so that it is capable of motion relative thereto substantially along the extent of the crank arm. The frame is formed to have transverse notches along one side of the slide path followed by the sliding block, which notches are engaged by detent means carried within the sliding block. The detent means are manually operable to release the detent from engagement with a particular notch on the adjustment frame so that the sliding block, with the pedal attached thereto, may be repositioned to a differing stroke radius, whereat the detent engages an appropriate notch. Additionally, the block may be locked into a particular radial position by a set screw acting through the block against the adjustment frame.
Preskitt also teaches several alternate embodiments. A first is similar to the preferred embodiment except that it employs a second detent mechanism engaging notches on the adjustment frame positioned to the opposing side of the sliding block from those of the preferred embodiment. Another alternate embodiment provides for substantially continuous radial adjustment by coupling the sliding block to a lead screw extending through the adjustment frame parallel with the extent of the crank arm. Rotation of the lead screw, which is journaled to rotate within the adjustment frame at either end, causes the sliding block to move either inwardly or outwardly, depending on the pitch direction of the lead screw and the direction of its rotation about its longitudinal axis. Yet another embodiment taught by Preskitt foregoes the use of a sliding block entirely by forming a plurality of spaced apart holes along the extent of the adjustment frame substantially parallel with the extent of the crank arm. Each of these holes is independently adapted to accept engagement of the pedal. In a further variation on the immediately preceding embodiment, Preskitt replaces the existing crank arm of the bicycle with a frame clamping directly to the crank shaft at its axis of rotation, this frame having a plurality of radially spaced apart holes for accepting engagement of the pedal at differing radii.
Where extension of the effective radius of a bicycle crank arm is desired, a device similar to that described in U.S. Pat. No. 4,850,245, issued Jul. 25, 1989 to N. Feamster, et. al. for "Bicycle Crank and Pedal Structure", may be employed. The Feamster device utilizes a sleeve configured to slidably telescope over the extent of the existing crank arm. The sleeve contains a plurality of holes therethrough, the most outwardly radially located hole being adapted to engage and support a pedal. In use, the sleeve is telescoped onto the crank arm until the desired extended pedal radius is established. Thereat, one of the plurality of radially inwardly spaced apart holes through the sleeve is aligned with the normal pedal engaging hole at the distal end of the crank arm. Bolt means are passed and engaged through this combined hole to secure the sleeve to the crank arm.
Without making specific reference thereto herein, the literature surrounding the present invention contains descriptions of many differing embodiments of devices useful in changing the pedal radius of a bicycle or similar apparatus. Those set forth hereinabove are believed to be the most relevant among the art which is presently known to the inventor hereof. In brief, the simplest known of such devices is the addition of blocks directly to the foot support portions of the pedals. While this approach accommodates user leg extension relative to the seat position, it does not appreciably alter the radius through which the foot moves in following the motion of the pedal. In each of such prior art devices, cited specifically herein or otherwise, closer concern should be given the examination thereof for their respective capabilities of accommodating differing ranges of motion to which a user may be limited, while also considering the ease with which changes in radius may be accomplished. Of further concern are such factors as ease of assembly, complexity of structure and design, and stability relative to off-set of the pedal from the extent of the crank arm. All such factors appear to vary greatly between the several references and other known approaches.